A conventional starter of this type is that shown in FIG. 1, wherein a DC motor 1 is constituted by a rotary shaft 2, an armature 3 attached to the rotary shaft 2, a permanent magnet 4 disposed in opposition to the armature 3 through a gap, and a yoke 5 for supporting the permanent magnet 4. A solar or spur gear 6 is formed on the rotary shaft 2 of the DC motor 1, a planet gear(s) 7 is engaged with the spur gear 6, and a support pin 8 for supporting the planet gear 7 is fixed to a flange 9 which is integrally fixed to an output rotary shaft 10. A ring-like internal gear 11 made of a resin material such as nylon or the like meshes with the outer circumference of the planet gear 7 so as to guide the planet gear 7. The internal gear 11 constitutes a planetary gear reduction device together with the spur gear 6 and the planet gear 7. A front bracket 12 is spigot-fitted to the yoke 5 of the DC motor 1. An intermediate bracket 13 has an outer circumferential portion engaged with a spigot portion 12a of the front bracket 12 and an inner circumferential portion arranged to support the output rotary shaft 10 through a sleeve bearing 14. A rubber ring 15 is mounted onto the spigot portion 12a of the front bracket 12 together with the internal gear 11 and the intermediate bracket 13 in such a manner that the rubber ring 15, the internal gear 11 and the intermediate bracket 13 are in close contact with the spigot portion 12a of the front bracket 12 in the axial direction and in close contact with each other in the radial direction. A thrust washer 16 is provided between the yoke 5 and the rotary shaft 2. A sleeve bearing 17 is provided between the rotary shaft 2 and the output rotary shaft 10, and another sleeve bearing 18 is provided between the planet gear 7 and the support pin 8. Steel balls 19 are provided between the armature rotary shaft 2 and the output rotary shaft 10 at their respective end portions so as to transfer a thrust load to each other. Helical splines 20 are formed on the output rotary shaft 10 at its outer circumferential surface and a not-shown overrunning clutch (including a pinion) is spline-fitted slidably axially.
Next, the operation of the above arrangement will be described. When the armature 3 is energized to generate rotary force, the rotary force is transmitted to the armature rotary shaft 2 and is further transmitted to the flange 9 through the spur gear 6, the planet gear 7, and the support pin 8, so that the rotational speed of the armature 3 is reduced by the planetary gear reduction mechanism and is transmitted to the output rotary shaft 10.
In the thus arranged conventional planetary gear reduction starter, since the internal gear 11 is made of resin such as nylon or the like, the internal gear 11 per se would be flexed when an impact is generated in starting an engine to thereby absorb the impact. However, the internal gear 11 is mounted onto the front bracket 12 in close contact with the latter radially as well as axially, so that even if an impact load due to a normal torque fluctuation in an engine is exerted on the internal gear 11, the amount of flexure of the internal gear 11 is so small that the normal impact load can not be absorbed.
In the case where the radial gap between the internal gear 11 and the front bracket 12 is made large, there has been a problem in that an engine is suddenly stopped in starting the engine, and when a pinion of the starter and a ring gear of the engine are not suitably engaged with each other so that the respective tooth-end surfaces of the former and the latter collide with each other, a large impact load is exerted on the internal gear 11 to transform the internal gear 11 in the radial direction too much to thereby brake the internal gear 11.
The present invention is achieved to solve the above-described problems, and therefore it is an object of the present invention to provide a planetary gear reduction starter in which when a normal impact load is exerted on an internal gear, the internal gear and an elastic body are effectively flexed to absorb the impact, while when an abnormally larger impact load is exerted on the internal gear, abnormal transformation of the internal gear is prevented to thereby prevent the internal gear from being broken.